Many geosynchronous orbit communication satellites currently in operation were designed with a finite amount of fuel and were not designed for the possibility of being refuelled. The design philosophy relied upon replacement of the satellites after they had exhausted the on-board fuel supply. In view of the expense of replacing satellites, it would be very advantageous to be able to refuel communication satellites which are either near their end of life, or have suffered an infant propulsion system failure, thereby extending their operational life by several years.
In many incidents, at the end of a satellite's 10 to 15 year life all of its subsystems are still functional and it is only the depletion of the carefully budgeted fuel load that drives retirement of the satellite. Using a current economic model, the ability to refuel 10 to 12 of these end of life satellites in one mission, would extend their useful life by 3 to 5 years and thereby delay the need to outlay the $150-$250 M to launch a replacement. Some satellites suffer from primary propulsion system failures soon after they are launched. In these cases the entire book value must be written off and compensation paid to the operator by the space insurer. The satellite becomes an asset of the space insurer and will eventually have to be disposed of in a graveyard orbit. If one of these assets can be refueled, extending its life by 5 to 10 years, most of the value of the spacecraft can be recovered.
The key technical difficulty is that these satellites were not designed for robotic servicing, and it is not generally accepted that such missions are technically possible. Specifically, most satellites are designed with fuel fill and drain valves that were intended to be filled once prior to launch and never opened or manipulated again. Thus, accessing these fill and drain valves remotely presents several major challenges and would involve several operations, each of which is difficult to accomplish robotically including: cutting and removal of the protective thermal blankets, removal of several lockwires hand wrapped around the valves, unthreading and removing outer and inner valve caps, mating the fuel fill line to the valve nozzle, mechanically actuating the valve, and when refuelling is complete, replacing the inner valve cap.
On-orbit servicing has been the subject of much study over the past thirty years. The idea of maintaining space assets rather than disposing of and replacing them has attracted a variety of ideas and programs. So far the concept has only found a home in the manned space program where some success can be attributed to the Hubble Space Telescope repair missions, Palapa-B2 and Westar rescue missions and the assembly and maintenance of the International Space Station.
Robotic capture and servicing of existing geostationary spacecraft has never been demonstrated. Over the past decade several of the key technologies required for orbital servicing have matured. These include autonomous rendezvous (ETS-VII (1998), XSS-11 (2005), DART (2006), Orbital Express (2007), autonomous docking (ETS-VII, Soyuz, Orbital Express), ground based robotic tele-operation (ETS-VII, SSRMS (2005), Orbital Express), and on orbit fluid transfer (ISS). However a gap exists in the technologies required to service or re-fuel an un-prepared satellite in orbit. An unprepared satellite is defined here as a spacecraft that was not designed to be manipulated or repaired by a robotic system. Some advances have been made in the technologies required to dock with an unprepared satellite, and both DLR (German Aerospace Center) and MDA have demonstrated through various R&D efforts that docking to a GEO communication satellite via the spacecraft's apogee kick motor is a viable docking option.
To date there have been no technologies disclosed that can solve the problem of accessing the fuel system of an unprepared satellite for the purpose of replenishing station keeping fuel. The majority of satellites in orbit today were not designed with orbital refuelling in mind and access to the fuel system is designed to be accessed by a human on earth before launch. The technologies required to access the target spacecraft's fuel system for the purposes of refuelling still have a very low technology readiness level, and are generally considered to be the main obstacle to a successful servicing mission.
United States Patent Publication No. 2006/0151671 (Kosmos) discloses an actuator arm mounted on a spacecraft designed as a servicing manipulator for use within a spacecraft service bay and includes an actuator arm connected to a base using flexible connection tapes.
United States Patent Publication No. 2006/0151671 discloses a servicing communication architecture in which the communication between a ground station and the servicing satellite is carried out via the communication system of the client satellites communication links. Also disclosed is a general servicing architecture in which target satellites are captured and returned to a servicing spacecraft. Within this servicing spacecraft it is proposed that any required servicing operations could be conducted.
The publication “On-Orbit Servicing by “HERMES On-Orbit-Servicing System, Policy Robust Planning”, C. Kosmos, American Institute of Aeronautics and Astronautics, SpaceOps 2006 conference proceedings”, pp 1 to 6, Apr. 26, 2006, discloses a satellite refuelling architecture that requires each satellite to be serviced to have a custom quick disconnect (QD) coupling attached to its service valve before launch. A preliminary design for a valve access tool used to access this valve is also presented.
Therefore, it would be very advantageous to provide a satellite refuelling system for earth-based controlled refuelling of unprepared satellites.